The demand for electric power has been diversified these years, and the need for reliable power supply and quick response to any changes is ever increasing. To meet this need, information on the conditions of power supply, such as line voltages and line currents, must be collected at a high speed through accurate measurement over a wide service area at as many locations as possible. The inventors noted that optical sensors, such as electro-optic elements, are suitable not only for accurate measurement but also for communication of a large amount of data at a high speed through the optical fiber communication systems.
Support insulators of electric apparatus which allow the use of optical are known, for example, Japanese Patent Laid-open Publication No. 143,619/1982 discloses an insulator having communication optical fiber disposed therein.
FIG. 16 shows an example of an insulator with an optical fiber as known in the prior art. An insulator body 81 has a hole 81a extending therethrough, and an optical fiber 84 with optical connectors 82, 83 mounted at opposite ends thereof is disposed in the hole 81a. For protection against moisture and dust particles, the hole 81a is sealed, for instance, by an O-ring 85 and filler 86.
The above-referred insulator has a shortcoming in that the sealed portion, such as the 0 ring 85 and the filler 86, is susceptible to deterioration due to external forces, temperature variations, moisture and aging. It has been difficult to keep the sealed portion intact over a long period of time in a stable manner. Thus, optical sensors connected to the optical fibers 84 in the insulator body 81 have experienced troubles due to insulation deterioration in the hole 81a or deterioration of the optical fiber 84 itself caused by moisture absorption or water infiltration. Further, the operation of inserting the optical fiber 84 in the hole 81 is cumbersome, and it has been a stumbling block for the quality control and mass production of the insulator.
Therefore, an object of the present invention is to provide a porcelain insulator with one or more optical transmission media, e.g., optical fibers, embedded therein, which insulator maintains excellent insulation and optical transmission over a long service life in a stable fashion, while eliminating the abovementioned shortcomings of the prior art under certain external conditions and aging.
FIG. 17 shows a circuit diagram of a conventional device for detecting and measuring the zero-phase-sequence current of a three-phase power line, and FIG. 18 shows a similar circuit diagram of the prior art for detecting and measuring the zero-phase-sequence voltage of the three-phase power line.
For zero phase-sequence current measurement, secondary windings of three current transformers 92 mounted o the three power line conductors 91 are connected in series by lead wires 93 and the serial circuit is closed by a resistor 94 as shown in FIG. 17. Voltage across the output terminals 95, 95 represents the sum of the zero phase-sequence currents in the three phase line conductors 91.
Under normal conditions, the currents in the three-phase power line conductors 91 are balanced, and the zero-phase-sequence current in the three phase power line conductors 91 is zero and the electric signal from the output terminals 95, 95 is also zero. When there is an unbalanced fault in the three-phase power line, the electric signal from the output terminals 95, 95 assumes a finite value depending on the type of the fault.
For the zero-phase-sequence voltage measurement, Y-connected primary windings of a three-phase potential transformer 96 are connected to the three-phase power line conductors 91 by lead wires 93, while the secondary windings of the potential transformer 96 are connected in an open delta which open delta is closed by a resistor 94 as shown in FIG. 18. The three-phase potential transformer 96 may be replaced with a combination of three single-phase potential transformers, provided that the windings are connected similarly.
The output voltage from the output terminals of the potential transformer 96, representing the voltage across the resistor 94, is the zero-phase-sequence voltage of the three-phase power line. The value of the zero-sequence voltage is zero as long as the line voltages of the three-phase power line are balanced, and it assumes a finite value when there is an unbalanced fault in the three-phase power line and the value of the zero-phase-sequence voltage depends on the type of the unbalanced fault.
One or both of the zero-phase-sequence current and the zero-phase-sequence voltage have been used for detection of fault in the three phase power line.
However, the conventional structure of insulator as shown in FIG. 17 and FIG. 18 has a shortcoming in that the lead wires 93 emanating from the current transformers 92 and the potential transformer 96 are susceptible to electromagnetic induction caused by the external electromagnetic field. Accordingly, the S/N ratio of the output signal is kept low and the accuracy of measurement is comparatively low. In order to improve the reliability of transmission lines and distribution lines, there is an increasing demand for accurate detection of those faults which have not been detected heretofore by conventional means, such as instantaneous groundings and intermittent groundings. Thus, there is a demand for higher accuracy of the measurement of line voltages and line currents.
Further the deterioration by aging of lead wires 93 can be a cause of grounding faults and when the insulating strength of the current transformer 92 and the potential transformer 96 are deteriorated, such transformers themselves can also be a cause of a grounding fault.